As shown in FIG. 10, a conventional speed change system for a bicycle provided with a front derailleur a and a rear derailleur b comprises a speed change operation assembly d1 for the front derailleur and another speed change operation assembly d2 for the rear derailleur. Each of the speed change operation assemblies is mounted at a suitable portion of a bicycle such as a handlebar and connected to a corresponding one of the derailleurs a, b by means of a corresponding control cable c1, c2. The derailleurs a, b are activated by operating the respective speed change operation assemblies d1, d2.
Each of the conventional speed change operation assemblies d1 , d2 includes a lever having a base portion rotatably supported on a fixed shaft which is mounted on the bicycle frame or handlebar. The control cable c1 , c2 is wound on the base portion of the lever. When the lever is turned in one direction, the control cable c1, c2 is wound further on a cable winding groove of the lever base portion. On the other hand, when the lever is turned in the opposite direction, the control cable c1, c2 is paid out from the winding groove under the tension applied by a return spring of the derailleur. The axial movement of the control cable is transmitted to the derailleur for actuation.
Such a derailler as shown in FIG. 10 (speed shifter of the external mounting type) is commonly used for laterally pressing a chain relative to a sprocket cluster which includes a plurality of diametrically different sprockets arranged side by side at a predetermined spacing, thereby performing a speed change by shifting the chain into engagement with a desired sprocket.
A rear derailleur, which is provided at the rear wheel, includes a chain guide which rotatably supports a guide pulley and a tension pulley, as disclosed in the Japanese Patent Application Laid-Open No. 63-251388 for example. The chain guide is rotatably supported, via a shift linkage mechanism such as a parallelogram pantograph mechanism, on a link base mounted on the bicycle frame while being urged in a chain tensioning direction. The shift linkage mechanism is deformed by operating a speed change lever assembly connected to the linkage mechanism via a control cable, thereby translating the chain guide, which is carried by a movable member of the shift linkage mechanism, axially of the hub shaft to shift the chain into engagement with a desired sprocket of a multiple freewheel for speed change.
On the other hand, a front derailleur includes a pair of inner and outer guide plates facing each other with the chain interposed therebetween. The pair of guide plates is moved laterally relative to a front gear, which includes a plurality of sprockets arranged side by side, for pressing the chain inward or outward into shifted engagement with a desired sprocket.
The conventional speed change operation assemblies described above are provided at a suitable portion of the handlebar or bicycle frame and may necessitate removing the hand from the handlebar at the time of performing a speed change. During the speed change operation, therefore, it becomes impossible to apply brake quickly, and steering of the bicycle is unavoidably instable since the bicycle must be steered by a single hand.
Meanwhile, a speed changes operation is required usually when the road condition change, for instance, from flat to slope or vice versa. In such a case, the running condition tends to become unstable with greater needs for braking.
In order to solve this problem, Japanese Utility Model Laid-Open No. 58-26693 proposes a so-called rotary-grip type speed change operating assembly which comprises a cylindrical operation member is supported around each grip end portion of a handlebar. The cylindrical operation member is rotationally operated to pull or release a speed change control cable.
The speed change operation assembly described in the above-mentioned publication makes it possible to perform a speed change operation without unhanding the handlebar. The running safety can be greatly improved.
Conventionally, the rear derailleur is designed to move the chain guide laterally inward for bringing the chain into shifted engagement with a lower speed sprocket (larger sprocket) when the control cable is pulled. On the other hand, the front derailleur is designed to move the pair of guide plates laterally outward for bringing the chain into shifted engagement with a higher speed sprocket (larger sprocket) when the control cable is pulled.
Therefore, for simultaneously causing both of the front and rear derailleurs to perform a change to a lower speed, it is necessary to wind the rear derailleur control cable by the relevant speed change operation assembly while paying out the front derailleur control cable from the relevant speed change operation assembly. As a result, it is necessary to turn the respective speed change operation assemblies for the front and rear derailleurs in the opposite directions for simultaneously causing both of the front and rear derailleurs to perform a change to a lower or higher speed.
However, a serious problem arises if such rotary-grip type speed change operation assemblies as disclosed in the above Japanese publication are used in combination with both of the front and rear derailleurs of the bicycle.
Specifically, with the conventional speed change system, the front derailleur is designed to bring the chain into shifted engagement with a higher speed sprocket, namely a larger sprocket, when the control cable is pulled, as described previously. On the other hand, the rear derailleur is designed to bring the chain into shifted engagement with a lower speed sprocket, namely a larger sprocket, when the control cable is pulled. Thus, it is necessary to turn the respective speed change operation assemblies, namely the cylindrical operations members rotatably supported on the handlebar, in the opposite directions for equally causing both of the front and rear derailleurs to perform a change to a lower speed at an abrupt change of the road condition to a steep uphill for instance.
However, it is an extremely unnatural action for the rider to turn the two cylindrical operation members in the opposite directions under the situation where the road condition is changing. Apparently, the rider's both hands gripping the handlebar are subjected to equally directed forces, consequently making it difficult to turn only one of the cylindrical operation members in a direction against the force. As a result, the rider is likely to erroneously turn both of the cylindrical operation members in the same direction, thus failing to perform a quick change to a desired lower speed by causing one derailleur to shift to a higher speed position.
Especially, with mountain bikes designed for off-road riding, a failure in speed change to follow an abrupt change of the road condition can cause a bicycle overturn due to insufficient transmission of a pedalling force to the wheel.